1. Field of the Invention
The invention relates to an apparatus and methods for exposing and transporting photographic film plates in a device for taking X-ray photographs in medical applications, and in particular the invention relates to a programmable high-speed film changer used for taking sequences of x-ray photographs.
2. Description of the Prior Art
Sequences of X-ray exposures are required in medical diagnostic techniques, such as in the making of angiograms. During such techniques it is necessary to expose a number of X-ray film plates in timed succession with possibly as many as four such plates being exposed in a second. The series of exposed X-ray photographs then provides a time record of a moving or pulsating organ such as the heart, or may show the progress of a radiopaque dye through tissue or an organ. The radiopaque dye is often quickly dispersed throughout the body's vascular system after injection. In particular the dye will enter and first flow through any particular tissue or organ, such as the brain, only during a predetermined time period, after which the dye becomes diluted and uniformly spread throughout the organ in question.
Therefore, it is necessary to expose a series of photographic plates to an X-ray image in rapid succession and during the predetermined time interval. The prior art has devised diagnostic X-ray equipment wherein photographic plates are serially transported from a cartridge into an exposure frame. When in the exposure frame, the photographic plate is sandwiched between two X-ray photoluminescent layers. The reaction of the X-ray photoluminescent layers to scattered X-rays from the tissue generates visible light which then exposes adjacent areas of the photographic film which are in contact with the X-ray photoluminescent layers. The exposed plate is then transported from the exposure frame into a storage cassette for removal and handling. The photographic sheets are generally large resilient plastic sheets, approximately 14" by 14". The plates must be pressed into close and intimate contact with the photoluminescent sheets in order to provide a high resolution and accurate image of the shadow cast by the X-ray beam.
The prior art design of such X-ray film changers has been susceptible to a number of defects and limitations. Firstly, the manner in which X-ray film is fed into the exposure plane and then pressed between the X-ray photoluminescent sheets often results in serious damage to the equipment in the cases where a multiple number of sheets are misfed into the exposure frame. Secondly, the receiving film cassette, which receives the exposed photographs plates, is typically operationally dependent upon gravity feed in order to stack a multiple number of exposed X-ray plates. Therefore, the spatial orientation of the X-ray equipment is limited and in certain orientations the equipment becomes inoperable. This is particularly disadvantageous when a series of X-ray exposures are taken through a range of angular orientations with respect to the patient. It is possible that in some portion of the range of angular orientations, the prior art X-ray film changer will jam with consequential serious mechanical damage to the changer.
Furthermore, in applications where two or more film changers are used to simultaneously or alternatively make X-ray exposures of a patient from two or more different angles during a single diagnostic run or x-ray exposure, the feed, exposure and removal of the X-ray plates within each film changer must be carefully synchronized with respect to each other. Lack of proper synchronization between two or more film changers will result in an erratic and haphazard series of X-ray photographs being taken which will not have the desired temporal relationship with respect to each other.
Therefore, the prior art has incorporated various means for synchronizing the operation of the transport motors within each of the film changers and has included various types of sensing devices in the design in order to verify synchronization of the film transport between the changers.
Finally, prior art X-ray film changers generally have a limited number of operational cycles available. In other words, the timing cycle of the film transport mechanism in prior art film changers is fixed to a selected multiple of minimum periods. Therefore, the rate of film exposure in any one changer or the offset times between two prior art changers are always restricted to a multiple of some minimum time period.
Furthermore, prior art changers are relatively heavy, typically weighing as much as 60 pounds, and entail a substantially complex collection of electromechanical components.
What is needed is a high-speed, X-ray film changer which is not subject to the above defects and limitations of the prior art. Specifically, what is needed is a reliable and inexpensive film changer capable of achieving high exposure rates, which is tolerant of film misfeeds, and which is flexibly programmable alone or in combination with other similar units without requiring complex measures to achieve synchronization.